The present invention relates to the grafting of polymers for purposes of obtaining cross-linked thermoplastics or elastomers, and more particularly the invention relates to grafting a silane compound upon the not yet cross-linked molecules of a polymer to obtain later cross-linking in the presence of moisture. Thus, the invention relates to the preparation of plastic or elastomeric material for subsequently obtaining cross-linking, the preparation being the grafting of silane upon the polymer. The plastic material so treated is to include olefinepolymerizates or olefine-mixed polymerizates and others, preferably polyethylene, and to be uased as jacket, envelope or cover for elongated material such as electrical cable, conductors or tubes, etc.
The German printed patent application DAS No. 1,794,028 discloses grafting alkoxy-silane components on organic polymerizates with subsequent curing in the presence of moisture to obtain cross-linking. An alkoxy-silane compound may have the structure R Si Y.sub.3 wherein R is a vinyl group or a gammamethacrylperoxipropyl group, and Y is an alkoxy group with less than six carbon atoms.
Particularly, this printed patent application discloses grafting of trimethoxy-vinyl-silane or triethoxyvinyl-silane or other functionally organic silanes on polyethylene macromolecule. The grafting of the silane compound is specifically obtained pursuant to extrusion in the presence of additives, which provide for suitable radicals such as peroxides, whereby the grafting requires rather high temperatures, such as from 180.degree. C to 220.degree. C. Prior to extrusion, silane and peroxide are mixed with granulated polyethylene. In fact, the polyethylene particles are surface coated with the liquidous mixture of silane and peroxide. Homogenization is produced later in the extruder, which also performs the grafting. The resulting product is granulated again, and the grafted, granulated copolymer is blended with polyethylene in a polyethylene batch which contains also a catalyst for cross-linking. The resulting final product is then form extruded e.g. it is extruded as a jacket around a cable, a tube or the like.
This known method has the disadvantage that the usual extruders produce local premature, cross-linking due to non-homogeneous peroxide distribution. The formation of such gel particles is particularly disadvantageous when the resulting product is to serve as electrical insulation. Thus, grafting will be carried out properly only if the temperature profile in the barrel of the extrusion process is adequately predetermined and controlled so that grafting proper occurs only after the materials have been homogenized sufficiently.
As stated, the coating of the granules introduces some inherent inhomogeneities in the distribution of peroxide, and premature cross-linking during cooling of the granulated grafting component is detrimental to this process. Therefore, cable insulation cannot readily be made in that manner particularly in those cases where electrical and mechanical properties are critical. The outline of the process above indicates further that a multiple of separate steps have to be performed which complicates the process. Also, the grafted but not yet cross-linked plastic can be stored only to a limited extent and for a limited duration as any moisture will start the cross-linking. It is apparent, however, that cross-linking should begin only after the plastic has assumed and has been given its final shape.
Another disadvantage of the known procedure is to be seen in that after the catalyst has been added (the grafting component was added earlier) extrusion becomes more complicated as long as even traces of moisture are present in the material. As a consequence, the surface of the product is quite rough which is a significant deficiency if the product is subjected to an electric field and voltage.